A non-contact digitizing control unit and a non-contact tracer control unit that trace a model surface and machine a material so that a contour is identical to that of the model by using non-contact distance detectors have been developed recently. In this case, optical distance detectors are utilized as non-contact distance detectors, which are fixed at the top of a tracer head, measure the distances to the model surface and calculate digitizing data; then the digitizing control unit machines after completion of the model surface tracing, while the tracer control unit simultaneously traces and machines by a cutter associated with the tracer head. Such machinings by the non-contact digitizing control unit or the tracer control unit are expected in the development of applications in the tracing and machining fields because a soft surface model can be used because no cautionary measures are taken to avoid damaging the model.
FIG. 1 is an explanatory drawing illustrating a laser beam from an optical distance detector, illuminated normal to a model surface, FIG. 2 is an explanatory drawing illustrating a laser beam illuminated oblique to the model surface, and FIG. 3 is an explanatory drawing illustrating scattered or reflected lights intercepted by the model.
In the non-contact digitizing control units of the prior art, there is a problem in that the accuracy of tracing and machining is reduced at the area where the slope angle of a model surface to the X-Y plane is large, because the distance to the model surface is measured from a determined direction to the model, e.g., Z-axis normal to the table surface (X-Y surface) on which the model is installed. That is, an optical measuring axis of the distance detector becomes oblique to the model surface in this area, therefore the spot light on the model surface is enlarged in the form of an ellipse, reduces the resolution of the detector, and decreases the tracing accuracy. This means the position of the image cannot be determined and the accuracy of the obtained distance data will be decreased when the image on the light sensing element becomes large, because the distance to the model surface can be obtained by converting the current into the distance; the current of which is generated because of the position of the image formed on the light sensing element of the optical distance detector.
The image on the light sensing element is the image formation of an optical spot on the model surface by means of an image formation lens, so that the image on the light sensing element also becomes large if an optical spot has a large area. The illuminated laser beam is normally not a beam but an optical bundle, therefore an optical spot on the model surface has a surface area, and because of that, the optical spot is a circle and is most appropriate when the model surface is nominal to an illumination measure shaft as shown in FIG. 1, while the optical spot is enlarged to an ellipse and the measuring accuracy is reduced when the model surface is oblique in relation to the illumination measure shaft as shown in FIG. 2.
Particularly, in the case of a distance detector by means of the triangulation method, the distance often cannot be measured because of an interference between the optical measuring axis and the model surface, and because of the interception of the reflection from an optical spot and scattered lights when the model angle becomes large as shown FIG. 3.
For this reason a method that comprises using 2 detectors simultaneously executing a plurality of measurements by a tracer head, selecting 3 arbitrary points, determining the normal vector of a model surface by these 3 points, rotating the distance detector to the direction of the normal vector, and improving the measuring accuracy can be considered. Japanese Patent Application No. 1-194500 is an example of this kind of non-contact tracer control unit.
According to the apparatus described, a tracer control unit capable of high accuracy distance measurements can be provided by a method that comprises obtaining each coordinate of each apex of a small square on the model surface from measured data previously and currently sampled when transferring along the tracing line of 2 non-contact distance detectors provided with the tracer head, determining the nominal vector by using the coordinates of 3 desired apexes among them, rotating the tracer head in the direction of projection (shadow) to the X-Y plane of the normal vector and, directing the measuring shaft of the non-contact distance detector to be as close to normal to the model surface as possible.
However, with the above mentioned apparatus, there are cases in which 3 points are on a direct line or nearly equal to the direct line when 3 points are arbitrarily selected among a plurality of points, so that the normal vector of a model surface cannot be accurately obtained. Namely, the nominal vector cannot be obtained when 3 points are on a direct line. Furthermore, as the surface curve change rates of the surface model become large when 3 points are nearly equal to the direct line, an approximate plane of the model surface far from the real model surface is obtained. Therefore, the attitude control of the distance detector is performed based on the inaccurate normal vector, so that there remains a problem in that a reduction in the accuracy of a tracing cannot be avoided.